I am sorry...
There are a few light-blue pixels between the red bar from your styler toolbar and the windowframe (on the left and right). It looks like the frame is wider where the styler toolbar is.

It is a often seen "error" in a setting in stylebuilder.
So you can correct it by goint into stylebuilder and then do what I said in my cryptic message before.
Ehhh hope I am making myself clear now, English is not my native language.

in the parts tree go to: values->general->rebar->band->
add/change the property "contentmargins" and set the value to 0,0,0,0